The separation and purification of hydrogen is practiced in a wide range of industrial applications. Typical applications include the cracking of ammonia and the subsequent separation of hydrogen from nitrogen using a hydrogen permeable membrane, the diffusion and collection of tritium from a fusion reactor and the removal of hydrogen from sodium in a nuclear reactor by the use of a cold trap.
A large number of physical and chemical hydrogen separation methods have been devised varying substantially in principle and in physical form. However, many are based on the use of some material in which hydrogen is very soluble or permeable or both.
Heretofore, one of the most attractive techniques for separating hydrogen from other gases involves the use of a heated membrane of palladium. Palladium is highly permeable to hydrogen but not to other gases so that a separation and purification can be effected by applying a pressure difference across the membrane and collecting pure hydrogen on the low pressure side. James B. Hunter describes such a process in U.S. Pat. No. 2,773,561. The high cost and low mechanical strength of palladium has resulted in many attempts to provide improved hydrogen permeable membranes. One such attempt is described by A.C. Markrides et al in U.S. Pat. No. 3,350,846. In this invention the high hydrogen permeability of Group V-B metals was recognized. The reason why these metals are impractical for use in the pressence of oxidizing gases was observed to be the rapid formation of an oxide film on the membrane surface which acts as a barrier to hydrogen entry or evolution. This defect was overcome by coating the Group V-B metals with palladium to prevent the formation of the oxide film.
Although this invention provided some improvement in cost compared to palladium, the high density and difficult fabrication problems of the Group V-B metals limited their application for hydrogen separation. These coated metals had the same limitation as palladium when used to separate hydrogen from fluids which are corrosive to palladium such as sodium or lithium. A means for providing a corrosion resistant membrane for use in sodium or other alkali metals at high temperature was described in my U.S. Pat. No. 3,622,303. In this invention pure nickel was used as a corrosion resistant, hydrogen permeable membrane. To prevent oxidation of the nickel on the surface where the hydrogen emerged, a thin coating of palladium was applied, which also produced a catalytic oxidation pumping mechanism.
Although this invention provided a corrosion resistant, hydrogen permeable membrane, the permeation rate of hydrogen and the yield strength of nickel were relatively low so that the pressure vessel design specification could not easily be met.
Another technique which is widely practiced for the removal of hydrogen from other fluids is known as "gettering". When using this process a material is selected which has a greater affinity for hydrogen than that of the substance from which the hydrogen is to be removed. By inserting the gettering material into the fluid, hydrogen can be absorbed by the getter. U.S. Pat. No. 2,926,981 to V.L. Stout et al described such a getter, based on alloys of titanium and zirconium. Zirconium is present in this alloy in the range of 10 to 98 atomic percent. These alloys were not provided with a surface coating, so they also absorbed other gases, greatly reducing the rate and quantity of hydrogen absorbtion. These alloys were not stablized in the body centered cubic crystalline structure.
In British Pat. No. 963,548, July 8, 1964, Pool and Sinclair disclosed a coated getter for removing hydrogen from alkali metals. Their getter material consisted of zirconium coated with nickel or a nickel based alloy. Zirconium exists in the hexagonal close packed crystalline form, which has a relatively low hydrogen diffusion coefficient. Consequently, it was necessary for Pool and Sinclair to limit the thickness of the getter to a maximum of 0.050 inches in order to achieve a practical rate of hydrogen absorbtion.
The behavior of titanium-zirconium alloys when hydrogen diffuses in them has been the subject of numerous scientific studies. John J. DeLuccia reported in "Electrolytic Hydrogen in Beta Titanium" in the Navel Air Development Center Report No. 76207-30, July 10, 1976. DeLuccia was interested in the diffusivity of hydrogen at ambient temperature using an aqueous solution in an electrolytic cell to generate hydrogen. He allowed hydrogen to diffuse into a palladium coated body centered cubic alloy of titanium-zirconium consisting of 11.5% Mo, 6% Zr, 4.5% Sn, balance Ti. He showed that at 21.degree. C. the palladium coating allowed hydrogen to diffuse into the alloy but that it accumulated in the body of the alloy in regions called "traps". The accumulated hydrogen caused catastrophic cracking and deformation. He found that part of this damage could be reversed upon heating or standing, but he did not attempt to diffuse hydrogen at higher temperatures to see if hydrogen trapping would persist in a heated alloy. The present invention is intended to overcome or at least mitigate the problems encountered with the prior art, as will become apparent as the description proceeds.